(a) Field of the Invention
The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery, and more particularly, to an electrolyte for a lithium secondary battery capable of preventing swelling of the battery when the battery is charged at room temperature, or when the battery is stored at a high temperature after charging, and a lithium secondary battery comprising the same.
(b) Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for batteries having high energy density for use as power sources in these portable electronic instruments. Lithium-transition metal oxides are often used as positive active materials for lithium secondary batteries, and lithium metals, lithium alloys, crystalline or amorphous carbons, or carbon composites are often used as negative active materials for lithium secondary batteries.
An average discharge voltage of a lithium secondary battery is about 3.6 to 3.7 V, which is higher than other alkali batteries, Ni-MH batteries, Ni—Cd batteries, etc. However, an electrolyte which is electrochemically stable in the charge and discharge voltage range of 0 to 4.2 V is required in order to generate such a high driving voltage. For this reason, mixtures of non-aqueous carbonate based solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc. are often used as electrolytes. However, such electrolytes have significantly lower ion conductivity than aqueous electrolytes which are used in Ni-MH batteries or Ni—Cd batteries, thereby resulting in the deterioration of battery characteristics during charging and discharging at high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxides of a positive electrode of a battery, are transferred to the carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium is reacted with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as a solid electrolyte interface (SEI) film. The SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbon negative electrode because organic solvents in an electrolyte with a high molecular weight solvate lithium ions and the solvent and the solvated lithium ion are co-intercalated into the carbon negative electrode.
Once the SEI film is formed, side reactions of the lithium ions with the carbon electrode or other materials are inhibited such that the amount of lithium ions is maintained. That is, the carbon of the negative electrode reacts with the electrolyte during the initial charging, thus forming a passivation layer such as a SEI film on the surface of the negative electrode such that the electrolyte solution is no longer decomposed, and stable charging and discharging are maintained (J. Power Sources, 51 (1994), 79-104). For these reasons, in a lithium secondary battery, an irreversible formation reaction of the passivation layer does not occur after the initial charging, and stable cycle life is maintained.
Thin prismatic batteries are problematic in that gases are generated inside such batteries when a carbonate based organic solvent is decomposed during the SEI film forming reaction (J. Power Sources, 72 (1998), 66-70). These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. The generation of gases can cause a battery to expand during charging. Furthermore, the passivation layer is slowly disintegrated by electrochemical energy and heat energy which increase with the passage of time when the battery is stored at high temperatures after it is charged. Accordingly, a side reaction between the exposed surface of the negative electrode and the surrounding electrolyte occurs continuously. Furthermore, the internal pressure of the battery increases with this generation of gas. The increase in the internal pressure induces the deformation of the prismatic battery and lithium polymer battery (PLI). As a result, regional differences in the cohesion between pole plates inside an electrode element (positive and negative electrode, and separator) of the battery occur, thereby deteriorating the performance and stability of the battery and making the mounting of the lithium secondary battery difficult.
As a method for solving the internal pressure problem, there is disclosed a method in which the stability of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting an internal electrolyte solution when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may be caused by an increase in internal pressure itself.
Furthermore, a method is known in which the SEI formation reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure. For example, Japanese Patent Laid-open Publication No. 97-73918A discloses a method in which the high temperature storage characteristics of a battery are improved by adding a diphenyl picrylhydrazyl compound of 1% or less to the electrolyte. Japanese Patent Laid-open Publication No. 96-321312A discloses a method in which cycle life and long term storage characteristics are improved using a N-butyl amine group compound of 1 to 20% in an electrolyte. Japanese Patent Laid-open Publication No. 96-64238A discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−3 M of calcium salt to the electrolyte. Japanese Patent Laid-open Publication No. 94-333596A discloses a method in which the storage characteristics of a battery are improved by adding an azo compound to inhibit the reaction between an electrolyte and a negative electrode of the battery.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as a SEI film by adding a small amount of organic or inorganic materials are used in order to improve the storage characteristics and stability of a battery. However, there are various problems with these methods: the added compound can decompose or form an unstable film by interacting with the carbon negative electrode during initial charging and discharging according to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in an electrode; and gas generated inside the battery can cause an increase in internal pressure, resulting in significant worsening of the storage characteristics, stability, cycle life, and capacity of the battery.